delara newsDelaware Amateur Radio Association, Delaware OH VOL 37 NUMBER 3
FINDING MY HF RFI PROBLEM
By Al Valentino, K8BRJThe factsSince about October 2017, I have been having an HF radio frequency interference (RFI) problem that has been almost always present (99% of the time). The RFI signal was very strong at about an S9 to +5 level (which meant it was nearby) and affected the 160-meter through 40-meter bands. Below is a screen shot of the RFI on the 80-meters band (when no other amateurs were present):The waterfall presentation above was set for about 7-seconds, which shows how the RFI repeated every 25KHz and each instance would swing back and forth across about a 5KHz range. Below is a close-up screen (set on 10KHz width) showing how the RFI vacillates back and forth:Isolating the SourceFirst Steps:Chuck WA8KKN instructed me to assure the RFI was not inside my home by:1.Shutting off the main breaker to my house.2.Put my HF transceiver on battery power.3.See if the noise goes away (it didn’t).NOTE: Looking back, it is important to identify the mode the RFI is most prevalent on. For example, my particular RFI was most troublesome on SSB and was almost unnoticeable on AM.Second Steps1.Identify which modes are most affected by the RFI.2.Obtain a portable Amateur LSB receiver.3.Make a portable directional antenna.4.Go hunting.5.If the source emanates from a neighbor, determine the best way to approach him.WHAT I DID1.THE RADIO: My first mistake was purchasing an AM/FM amateur receiver. I quickly realized AM mode was inadequate. I subsequently bought an amateur receiver (Tecsun PL-880) that had the SSB features and proved to be exactly what I needed.2.THE ANTENNA: Chuck WA8KKN instructed me to build an antenna from a simple circle of stiff wire between 12 and 24 inches and attach it to a broom handle. I used some heavy wire I got from Bob W8ERD and tied it to the broom handle. Below is the result, which also shows my PL-880 (the black box) attached to it:3.After dark using the above rig, I walked around my home and the nearby alley & street. It soon became evident that the strongest signal was emanating from my neighbor’s home immediately north of me.4.My next challenge was how to approach my neighbor. Ken W8SMK and Dave W8KFJ gave me some excellent approaches because I was not very familiar with my neighbor and did not want to offend him.After talking with my neighbor and building good rapport, he let me scan his property. It soon became evident that the source was emanating somewhere in the vicinity of his Generac generator in an approximate 6-foot square area.I explained the results to him and he said he would have his regular generator maintenance service look into the problem. Two days later the RFI disappeared. I waited and additional three days to assure the RFI was truly gone and gave my neighbor a call. He told me Spectrum cable TV found the problem. He was having problems with his Spectrum cable reception and called them to look into the problem. The Spectrum technician found a TV coax cable (behind the generator) gnawed by a squirrel or some other varmint. He fixed the cable and the RFI disappeared.Below are some other photos of my rig:Tom Thompson (W0IJV), wrote a wonderful RFI article in the November 2014 issue of “QST.” In the article he said:The Loop AntennaA loop antenna is bidirectional. You can rotate the loop easily if you hold it above your head with the feed point at the bottom. In this position, vertically polarized signals peak when the loop is turned edgewise to the source. Additionally, a deep null in the signal occurs for vertically polarized signals when the loop is turned 90 degrees from the peak position. In other words, the signal is nulled when looking through the loop toward the source. That sharp null is most useful for determining the direction of the noise source since most consumer RFI seems to be vertically polarized as long as you are further than about one hundred feet from the source. In the article referenced above, Tom Thompson (W0IJV) also gives precise instruction on how to use this antenna. This article can be downloaded from the ARRL website.